The amplifier described here is an update of the High Power linear amplifier described in the July,
1995 issue of the "Radiolubitel. KW i UKW" magazine in Belarus. In English, this is "Radio Amateur.
HF and VHF". The author, EW1MM Gary Podgorny (callsign in the 1970’s was UA6LFC) constructed his
1st high power amplifier in 1976 using GS-7B metal-ceramic triode (see the black and white picture
at left - click for zoom). The GS-7B metal-ceramic triode provided the same power output as
the GS-35B because the parameters of both tubes are the same. It looks like "GS-7B" might be the
name in the 1970’s of the "GS-35B" today.

In the color picture at right (click for zoom) one can see the latest model of EW1MM project.
The modified schematic diagram of the amplifier can be seen in Fig.1 above. The Russian high-mu
triode, a GS-35B, is connected in a grounded-grid configuration which provides about the most simple
layout possible. Although the input impedance of the grounded-grid GS-35B is close to 50 ohms and
would provide a good match for a driver with fixed-impedance output, a pi-network input circuit would
be recommended for better linearity and to lower the driving power requirements.

The exact value of the shunt resistor is different and depends on the PIV of the diodes. R=PIV
rating of the diode should be multiplied by 500 Ohm. Example: The PIV of the diodes are 1000.
1000 x 500=500 000 Ohm (500 k), so each diode should be shunted by 500 k resistor.

The secondary side of the power supply is conventional. The power supply provides approximately
3200 volts under no signal conditions and 3000 volts at a cathode current of 1 ampere. C23 and
C24 are filter capacitors. The total value is 50 mF, 6000 V. I would not recommend using the
electrolytic capacitors here. A high-voltage meter is included in the power supply. A low
voltage power supply is shown in Fig.3 below.

The linear amplifier operates in a grounded-grid circuit. The GS-35B amplifier requires a driver
that can deliver at least 80 watts p-e-p. Actually, it is best to use a driver that is capable
of 100 watts p-e-p, to assure that sufficient driving power is available on 21 and 28 MHz, the
frequencies at which the efficiency of coupling circuits is often poor in comparison to that of
the lower bands.

The bias of the amplifier is provided with D1…D7 – D815A, 5.6 V, 8 W zener diodes (Use a heat sink).
It is a good idea to use one of the well known adjustable Bias Circuit with high power NPN transistor
or something like that instead of D1…D7. The bias voltage for GS-35B is between 28…35 V.In standby
mode, the amplifier is biased to cutoff (near-zero cathode current) by the D8 – D817A, 56 V, 5 W zener
diode.

The grid current indicators are a portion of the grid-trip overload circuit. When grid current
exceeds a preset value, grid current is drawn through the 10 Ohm resistor (R1, Fig.1) and the
voltage drop across it turns on transistors VT1 and VT2 which operate the red and green LEDs.
Trimpots R3 and R6 provide the means to select grid current levels at which the LEDs are activated.
In this amplifier, the Green LED turns on when the grid current is 30% of max cathode current, and
the Red one at 32…33% of max cathode current. Any flashing of the Red LED indicates excessive drive
power or improper plate circuit loading.

A grid-trip module is included in the amplifier to protect the tube against excessive grid current.
Again, high levels of grid current can be caused by excessive drive, improper plate circuit loading
or lack of a proper load on the amplifier. When grid current exceeds 35% of max cathode current,
relay K4 is tripped (Fig.1), so the amplifier will go to the BYPASS position. Pushing the "Reset"
(Fig.1) button places the amplifier back in service. The grid-trip protection circuit is simple
but it works very well.

It is dangerous practice to place the plate current meter in +3000 lead. The meter here is placed
in the Cathode return circuit, so it reads the Cathode Current, which is the sum of the Grid and
Plate Currents.

Grid current is monitored by measuring the voltage drop across a 10 Ohm, R1, 2 watt (see Fig.1 above)
resistor through which grid current passes.

The Grid current of the GS-35B with full drive power and good SWR in the antenna system can reach
maximum 30% of the Cathode current. The max data sheet of GS-35B cathode current is 1.4 A. So
the conditions are absolutely normal when working cathode current is 1 A, the grid current is not
more than 300 mA, plate voltage is 3000 V under the load, SWR in the input and in the output of
the Power Amplifier is around 1.2:1 or better.

In this amplifier the tube is placed in the horizontal position, but it can also be placed vertically
with no problem. The cooling system provides cooling air in the direction from anode of the tube
to the heater.

The output tank circuit is a pi-network with air variable capacitors. If adjustments are correct
in the output tank, maximum grid current, minimum plate current, and maximum power output take
place at the same time. The 2.5 mH RF choke is connected between the output end of the pi-tank
and ground to prevent +3000 V from appearing at the antenna terminals should C2 develop a short.

The air flow system must always be used when any power is applied to the GS-35B – even filament.
The RF drive power should never reach the GS-35B unless the tube has plate voltage applied.